DISEASES OF PLANTS. 403 



pwrples, or pepper-corn, is a disease afieeting especially the grains of 

 wheat. The infected grains 'become first of a dark green, and ulti- 

 mately of a black colour. They become rounded like a small pepper- 

 corn, but with one or more deep furrows on their surface. The glumes 

 spread,open, and the awns become twisted. The blighted grains are 

 full of a fine white cottony matter, which, when moistened and put 

 under the microscope, is seen to consist of a multitude of minute in- 

 dividuals of the Vibrio tritici, or eel of the wheat. The animalcules 

 deposit their eggs in the ovary, and their young are hatched in eight 

 or ten days. Henslow calculates that 50,000 of the young might be 

 packed in a moderately sized grain of wheat. The Vibrio retains its 

 vitality long. It will remain in a dry state for six or seven years, 

 and when moistened with water will revive. The wheat-fly, or 

 Cecidomyia tritici, is another destructive insect. It deposits its eggs 

 by means of a very long retractile ovipositor, and is seen abundantly 

 in warm evenings. The Cecidomyia destructor, or Hessian fly, also 

 causes injury, and is said to be very destructive to wheat in America. 

 These insects are destroyed in numbers by the Ichneumons, which 

 deposit their ova in their bodies. The Apple-tree mussel, or dry-scale 

 Aspidotus conchiformis, attacks the bark of Apples, Pears, Plums, 

 Apricots, and Peaches. Many of the Coccus tribe are highly injurious 

 to plants. One of this tribe, ia 1843, destroyed the whole orange 

 trees in the island of Fayal, one of the Azores. Many insects cause 

 the rolling up of leaves. Tortricida viridana acts thus on the leaves 

 of the Oak, and various species of Losotsenia do so with other trees. 

 Adelges abietis is the aphis which causes the leaves of the Spruce-fir 

 to be united together, so as to have the appearance of a cone. 



The insect called the Cofifee-borer {Xylotrichus quadrupes) perforates 

 the wood of the Oofiee-tree and destroys the plant. The recent Vine- 

 disease has been caused by Phylloxera vastatrix, a very minute 

 Homopterous insect, not more than -}-, inch in length. It has a pro- 

 boscis lying in a groove on its under side, and with this it pierces 

 the roots on which it feeds. It draws nourishment by means of a 

 sucker. The insect is yellow in summer, but in autumn it turns 

 to a copper-brown. Its active life is from the beginning of April to 

 about the month of October ; it hibernates during the other months. 



Many insects, called miners, make their way into the interior of 

 leaves, and hollow out tortuous galleries, sometimes causing an alter- 

 ation in the colour of the leaves. Galls are caused by the attacks of 

 species of Cynips, which are provided with ovipositors, by means of 

 which they pierce the bark or leaves, with the view of having a nidus 

 for their ova. These galls are very common on the Oak, and are 

 called oahapples. Sometimes they have one cavity, at other times 

 they are divided into numerous chambers, each containing a grub, 

 pupa, or perfect fly, according to the season. GaUs are produced on 



